1. Field of the Invention
The present invention relates to a recording head of an image recording apparatus, and in particular, to a recording head in which a large number of recording elements, such as LEDs, for generating recording energy are linearly arranged. The recording head of the present invention is preferably used as a recording head in an image recording apparatus, such as a copying machine of an electrophotographic recording system, a facsimile machine and a printer.
2. Related Background Art
Conventionally, in an image recording apparatus for performing image recording by using the electrophotographic recording system, an LED recording head is used. In the LED recording head, a large number of LED light-emitting recording elements, which are respectively radiated in accordance with recording information, are linearly arranged. As a driving system for driving the LED recording head, a matrix driving system is utilized since this driving system can use an inexpensive driving IC.
Such a recording head of a matrix-driving system is disclosed in Japanese Patent Laid-Open Application Nos. 4-348962, 8-156325 and 8-187889, for example.
In a conventional LED recording head of a matrixdriving system, an LED chip including a plurality of LED devices is die-bonded on a glass epoxy substrate, a matrix-wiring pattern is formed on the substrate and the wiring pattern is wire-bonded to an electrode pad of each LED device.
FIG. 1 illustrates an equivalent circuit of the LED recording head of a matrix-driving system. The recording head includes fifty-six (56) LED chips 2-1 to 2-56 which are linealy arranged. Each of the LED chips 2-1 to 2-56 respectively includes sixty-four (64) LED elements 2-1-1 to 2-1-64 or the like. In each LED chip, LED devices are linearly arranged, and the arrangement direction of those LED devices is coincident with the arrangement direction of the LED chips. The arrangement pitch of the LED devices is approximately 12 pieces/mm, and the recording head is capable of performing recording of an A3 size with a density of 300 DPI (dots per inch).
Anodes of N-th (N=1 to 64) LED devices in the respective LED chips are connected to a common wire and are driven with a constant current by anode drivers 1-1 and 1-2. On the other hand, cathodes of sixty-four (64) LED devices in each LED chip are connected in the chip and are succesively driven in a time-sharing manner by cathode drivers 3-1 to 3-6. The cathode driver is constructed by six (6) chips each sharing ten (10) channels. Therefore, the matrix-driving of 64.times.56 can be performed.
FIG. 2 illustrates a plan view of the LED chip 2-1. In the LED devices 2-1-1 to 2-1-64, anode terminals of odd-numbered devices extend to an opposite side to those of even-numbered devices (i.e, the lower and upper sides in FIG. 2 respectively), and wire bonding pads (WBPs) 4-1-1 to 4-1-64 are connected to end portions of the respective anode terminals. Therefore, the arrangement pitch of the WBPs is approximately 6 pieces/mm, and a direct wire-bonding between the WBPs and the wiring pattern formed on an ordinary glass epoxy substrate can be executed.
FIG. 3 illustrates a schematic plan view of the above-discussed LED recording head. Fifty-six (56) LED chips 2-1 to 2-56 are die-bonded to a central portion of a glass epoxy substrate 5 with respect to a lateral direction and are arranged along a longitudinal direction of the substrate 5. In regions 15 on both sides of those LED chips, matrix-wiring patterns are formed. The matrix-wiring patterns are is connected to the WBPs of anode terminals of the LED devices in each LED chip by bonding wires (see FIG. 1). At opposite end portions of the substrate 5 with respect to the longitudinal direction, the above-discussed anode drivers 1-1 and 1-2 are respectively mounted. One anode driver 1-1 is connected so as to drive the above-discussed odd-numbered LED devices, and the other anode driver 1-2 is connected so as to drive the above-discussed even-numbered LED devices. That is, though the anode drivers 1-1 and 1-2 are represented as a unit for sixty-four (64) channels in FIG. 1, as a matter of fact those anode drivers 1-1 and 1-2 are respectively located at different positions with each sharing thirty-two (32) channels. Further, cathode terminals of each LED chip extend to a side of the bottom surface of this LED chip (i.e, the surface of the LED chip which is connected to the glass epoxy substrate 5), and those cathode terminals are connected to cathode drivers 3-1 to 3-6 each sharing ten (10) channels.
In such a conventional LED recording head as illustrated in FIG. 3, the width of the glass epoxy substrate 5 is approximately 20 mm, and this value is disadvantageous to a recording apparatus which is intended to be made compact by using a small photosensitive drum.
It is possible to obtain a smaller recording apparatus by forming the matrix-wiring as a multi-layer structure, but such a structure is complicated, hence the number of manufacturing steps and cost are increased. Those facts are disadvantageous to the recording apparatus.